Negative pressure booster

ABSTRACT

In a negative pressure booster ( 1 ) of the present invention, at the time of performing a usual braking operation in a low deceleration region, a force attributed to the pressure difference between pressures applied to a variable pressure chamber and a constant pressure chamber is equal to or smaller than a sum of a set spring load of a spring ( 31 ) and a spring load of a valve spring ( 18 ). Accordingly, a vacuum valve seat member ( 27 ) is not moved and the usual braking operation is performed at a small servo ratio. Further, during the usual braking operation in an intermediate deceleration region, the force attributed to the pressure difference is larger than the sum of the above-mentioned spring loads and hence, the vacuum valve seat member ( 27 ) moves rearwardly while pushing a valve element ( 12 ). Accordingly, a valve opening quantity of an atmospheric valve ( 16 ) is increased while shortening a pedal stroke and hence, a braking operation for intermediate deceleration is performed at a larger servo ratio, whereby brake manipulation feeling can be enhanced.

BACKGROUND ART

The present invention relates to a technical field of a negativepressure booster which is used in a brake booster or the like, and moreparticularly to a technical field of a negative pressure booster whichis used as a brake booster or the like of a brake system in a vehiclewhich can obtain a deceleration during the usual braking operation of avehicle having a large vehicle weight corresponding to a pedal strokeamount.

Conventionally, in a brake system of a vehicle such as an automobile, anegative pressure booster which makes use of negative pressure has beenused in a brake booster. In such a conventional generally-used negativepressure booster, the interior of the negative pressure booster ispartitioned into a constant pressure chamber into which negativepressure is introduced during the usual operation using a power pistonand a variable pressure chamber in which the pressure varies. Further,during the usual braking operation with a usual step-in motion of abrake pedal, when an input shaft advances, a control valve is changedover and the atmosphere enters the variable pressure chamber. Then, thepressure difference arises between the variable pressure chamber and theconstant pressure chamber and hence, the power piston is advanced.Accordingly, the negative pressure booster boosts an input of an inputshaft attributed to a pedal step-in force at a given servo ratio andoutputs the boosted force. Due to this output of the negative pressurebooster, a master cylinder generates master cylinder pressure and awheel cylinder is operated with the master cylinder pressure thusperforming the usual braking operation.

Here, with respect to a vehicle such as a one box car or a recreationalvehicle, recently, there has been observed a trend that a vehicle weightor a carrying weight is increased. Accordingly, in such a vehicle, alongwith the increase of the vehicle weight or the carrying weight, a brakemanipulation quantity (a pedal stroke quantity) which becomes necessaryduring the usual braking operation is increased. In this manner, thebrake manipulation quantity of a driver during the usual brakingoperation is increased and hence, it is difficult to achieve thefavorable brake feeling.

On the other hand, in Japanese Patent Laid-open 2001-341632, there hasbeen proposed a negative pressure booster which can obtain a largeoutput even with a small pedal step-in force, that is, even with a smallinput and can perform a brake assist (hereinafter also referred to as“BA”) operation in case of emergency. According to the negative pressurebooster disclosed in Japanese Patent Laid-open 2001-341632, even in acase that a moving speed of an input rod corresponding to the a pedalstep-in speed is larger than a corresponding moving speed during theusual braking operation, when an input which is applied to the input rodis smaller than a given value, the BA operation is not performed, andonly when the input which is applied to the input rod becomes equal toor above the given value, the BA operation is performed. That is, thenegative pressure booster is configured to generate an output at thetime of performing the BA operation which is larger than an outputgenerated during the usual braking operation in response to the sameinput. In other words, at the time of performing the BA operation, thenegative pressure booster can generate the larger output even with theinput smaller than the input during the usual braking operation. Here, astroke of an input rod of the negative pressure booster is shortenedcompared to a stroke when the large output can be obtained with theoutput equal to the output during the usual braking operation.

Further, in Japanese Patent Laid-open Heill (1999)-278245, there hasbeen also proposed a technique which sets a servo ratio at a small valueduring the initial phase of the operation of the negative pressurebooster and sets the servo ratio at a large value during the later phaseof the operation of the negative pressure booster. In the negativepressure booster disclosed in the Japanese Patent Laid-open Heill(1999)-278245, a reaction mechanism includes a reaction disc, a springand resilient means, wherein during the usual braking operation, areaction is transmitted to a valve plunger by way of a reaction discduring the initial phase of the braking operation so as to decrease theservo ratio, and the reaction is transmitted to the valve plunger by wayof the reaction disc and the spring during the later phase of theoperation of the negative pressure booster thus increasing the servoratio. Further, due to the provision of the reaction disc and theresilient means, hysteresis which exhibits the different outputs of thenegative pressure booster between at the time of stepping in the brakepedal and at the time of releasing the step-in operation is obtained.Due to this hysteresis, the braking feeling is enhanced.

Here, it is considered that by applying the above-mentioned negativepressure booster disclosed in Japanese Laid-open Patent 2001-341632 to avehicle which requires the deceleration higher than the decelerationduring the usual braking operation, the large deceleration is obtainablewith the small pedal step-in force. In this case, since the pedal strokecan be shortened, the brake feeling can be enhanced.

However, in the negative pressure booster disclosed in Japanese PatentLaid-open 2001-341632, the BA operation is performed only in the quickstep-in operation in which the pedal step-in speed is faster than thestep-in speed during the usual braking operation and, at the same time,the pedal stroke shortening function is performed. Accordingly, unlessthe pedal step-in speed is fast, the pedal stroke is not shortened andhence, it is difficult to obtain the favorable brake feeling. Further,there may arise a drawback that an operation sound is generated due tothe engagement or the disengagement of an engaging portion of the BAmechanism.

Further, in the above-mentioned negative pressure booster disclosed inJapanese Patent Laid-open Heill(1999)-278245, a servo ratio is set at asmall value during the initial phase of the braking operation within theusual braking operation region and the servo ratio is set at a largevalue in the later stage of the braking operation. Accordingly, in thisnegative pressure booster, no consideration is made with respect to thebrake system of the vehicle which requires the deceleration higher thanthe deceleration during the usual braking operation. Further, in thisnegative pressure booster, although the brake feeling is enhanced due tothe hysteresis between the step-in time of the brake pedal and the brakepedal release time, no consideration is made with respect to thedeterioration of the brake feeling due to an increase of the pedalstroke at the time of performing the operation with the highdeceleration.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a negative pressurebooster which can enhance the manipulation feeling by shortening astroke of an input member in an output region which exhibits an outputlarger than a given output and, at the same time, has the simplerstructure, can be easily assembled, and can be easily manufactured at alow cost.

To achieve such an object, a negative pressure booster of the presentinvention includes at least a valve body which is reciprocally disposedin the inside of a shell, a power piston which is mounted in the valvebody and partitions the interior of the shell into a constant pressurechamber into which negative pressure is introduced and a variablepressure chamber into which atmosphere is introduced at the time ofoperating the negative pressure booster, a valve plunger which isconnected to an input shaft and is slidably disposed in the inside ofthe valve body, a vacuum valve which controls the communication or theinterruption between the constant pressure chamber and the variablepressure chamber, and an atmospheric valve which controls theinterruption or the communication between the variable pressure chamberand at least the atmosphere due to the operation of the valve plunger,in which a stroke shortening mechanism is provided which shortens amanipulation stroke quantity of the input shaft in an output region withan output larger than a given output than the manipulation strokequantity of the input shaft in the output region with the output largerthan the given output when the manipulation stroke quantity of the inputshaft is changed at a change rate of the manipulation stroke quantity ofthe input shaft with respect to the output in an output region with anoutput equal to or below the given output.

Further, the present invention is also characterized in that the strokeshortening mechanism is atmospheric valve opening quantity increasingmeans which is operated in the output region with the output larger thanthe given output and increases a valve opening quantity of theatmospheric valve larger than the valve opening quantity during theusual operation, and the operation of the atmospheric valve openingquantity increasing means is controlled in response to pressurecorresponding to the input.

Further, the present invention is also characterized in that thepressure which controls the operation of the atmospheric valve openingquantity increasing means is pressure of the variable pressure chamber.

Further, the present invention is characterized in that the vacuum valveincludes a valve element and a vacuum valve seat on which the valveelement is detachably seated and, the atmospheric valve includes thevalve element and an atmospheric valve seat on which the valve elementis detachably seated, and the atmospheric valve opening quantityincreasing means includes a valve seat member which has a vacuum valveseat mounted on one end side thereof, wherein the valve seat member ismounted in the valve body movably between a first position which ispositioned in the output region with the output equal to or below thegiven output and a second position which is positioned in the outputregion with the output larger than the given output, and the movement ofthe valve seat member is controlled in response to the pressure of thevariable pressure chamber.

Further, the present invention is characterized in that the movement ofthe valve seat member is controlled in response to the pressuredifference between the variable pressure chamber and the constantpressure chamber.

According to the negative pressure booster of the present inventionhaving such a constitution, with the use of the stroke shorteningmechanism, in the output region which exhibits the output larger thanthe given output, it is possible to shorten the manipulation strokequantity of the input shaft than the manipulation stroke quantity of theinput shaft in the output region which exhibits the output larger thanthe given output when the manipulation stroke quantity of the inputshaft is changed at a change rate of the manipulation stroke quantity ofthe input shaft relative to the output in the output region where theoutput is equal to or below the given output. Accordingly, even when theoutput becomes larger than the given output during the usual brakingoperation, it is possible to obtain the favorable manipulation feelingwithout increasing the stroke of the input shaft.

Further, according to the negative pressure booster of the presentinvention, in the output region which exhibits the output equal to orbelow the given output, the atmospheric valve opening quantityincreasing means which constitutes the stroke shortening mechanism isnot operated, while in the output region which exhibits the output equalto or below the given output, the relatively small output is generated.Further, in the output region which exhibits the output larger than thegiven output, the atmospheric valve opening quantity increasing means isoperated to set the valve opening quantity of the atmospheric valvelarger than the valve opening quantity in the output region whichexhibits the output equal to or below the given output and hence, in theoutput region which exhibits the output larger than the given output, itis possible to generate the relatively large output. Here, since theoperation of the atmospheric valve opening quantity increasing means iscontrolled in response to a pressure corresponding to an input from theinput shaft, it is possible to operate the atmospheric valve openingquantity increasing means without being influenced by the stroke of theinput shaft. Accordingly, it is possible to generate the large outputwithout increasing the stroke of the input shaft whereby the favorablemanipulation feeling can be obtained.

Further, the operation of the atmospheric valve opening quantityincreasing means is controlled in response to the pressure correspondingto the input applied to the input shaft and hence, it is possible toeliminate mechanical engaging means whereby the structure of theatmospheric valve opening quantity increasing means can be simplified.Further, the atmospheric valve opening quantity increasing means can beeasily assembled and can be manufactured at a low cost. Still further,since the atmospheric valve opening quantity increasing means isoperated under a pressure control, the generation of an operation soundwhich is generated at the time of performing the operation can besuppressed.

Still further, according to the present invention, the operation of theatmospheric valve opening quantity increasing means is controlled inresponse to the pressure of the variable pressure chamber and hence, thepressure of the variable chamber can be directly utilized. Accordingly,the structure of the atmospheric valve opening quantity increasing meanscan be further simplified thus further facilitating the assembling ofthe atmospheric valve opening quantity increasing means.

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 is a cross-sectional view showing an example in which anembodiment of a negative pressure booster according to the presentinvention is applied to a brake booster in an inoperative state;

FIG. 2 is a partially enlarged cross-sectional view showing portions ofa vacuum valve and an atmospheric valve in FIG. 1 in an enlarged manner;

FIG. 3 is a partially enlarged cross-sectional view similar to FIG. 2and shows a state at the time of performing a usual braking operationand a state at the time of performing the intermediate and highdeceleration using the negative pressure booster of the example shown inFIG. 1 in a partially enlarged manner;

FIG. 4 partially shows the state of the negative pressure booster of theexample shown in FIG. 1, wherein FIG. 4(a) is the view showing aninoperative state, FIG. 4(b) shows a state during the usual brakingoperation, and FIG. 4(c) shows the state at the time of performing theintermediate or high deceleration;

FIG. 5 explains an operation of a vacuum valve seat member in thenegative pressure booster of the example shown in FIG. 1, wherein FIG.5(a) is a view showing the state at the time of performing theintermediate or high deceleration, and FIG. 5(b) is a view showing adynamic equivalent state in FIG. 5(a);

FIG. 6 is a view showing input/output stroke characteristics of thenegative pressure booster of the example shown in FIG. 1; and

FIG. 7 is a view showing input/output characteristics of the negativepressure booster of the example shown in FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, best modes for carrying out the present invention areexplained.

FIG. 1 is a cross-sectional view showing an example in which anembodiment of a negative pressure booster according to the presentinvention is applied to a brake booster in an inoperative state, andFIG. 2 is a partially enlarged cross-sectional view showing portions ofa vacuum valve and an atmospheric valve shown in FIG. 1 in an enlargedmanner. Here, in the explanation made hereinafter, “front” and “rear”indicate “left” and “right” in respective drawings.

First of all, in the negative pressure booster of this embodiment,constitutional parts which are equal to corresponding constitutionalparts of a conventional general negative pressure booster are explainedbriefly. In FIG. 1 and FIG. 2, numeral 1 indicates the negative pressurebooster, numeral 2 indicates a front shell, numeral 3 indicates a rearshell, numeral 4 indicates a valve body, numeral 5 indicates a powerpiston, numeral 6 indicates a power piston member, numeral 7 indicates adiaphragm, numeral 8 indicates a constant pressure chamber, numeral 9indicates a variable pressure chamber, numeral 10 indicates a valveplunger, numeral 11 indicates an input shaft, numeral 12 indicates avalve element, numeral 13 indicates a vacuum valve seat, numeral 14indicates an atmospheric valve seat, numeral 15 indicates a vacuumvalve, numeral 16 indicates an atmospheric valve, numeral 17 indicates acontrol valve, numeral 18 indicates a valve spring, numeral 19 indicatesan atmosphere introducing passage, numeral 20 indicates a vacuumpassage, numeral 21 indicates a key member, numeral 22 indicates adistance member, numeral 23 indicates a reaction disc, numeral 24indicates an output shaft, numeral 25 indicates a return spring, andnumeral 26 indicates a negative pressure introducing passage.

Here, in the same manner as the conventional general negative pressurebooster, the output shaft 24 operates a piston of a master cylinder.

Next, the constitution of characterizing parts of the negative pressurebooster 1 of this embodiment which are different from conventional partsare explained.

As shown in FIG. 2, in the negative pressure booster 1 of thisembodiment, a vacuum valve seat member (corresponding to a valve seatmember of the present invention) 27 is slidably fitted in an inner hole4 b formed in the valve body 4 in the axial direction, while theabove-mentioned vacuum valve seat 13 is formed on a rear end of thevacuum valve seat member 27. Accordingly, the vacuum valve seat 13 isalso movable relative to the valve body 4. Further, an annular flange 27a which projects inwardly is formed on a front end of the vacuum valveseat member 27.

Then, due to the provision of a sealing member 28 such as a cup seal orthe like which is mounted on an outer peripheral surface of the vacuumvalve seat member 27, an interface between an inner peripheral surfaceof an inner hole 4 b of the valve body 4 and an outer peripheral surfaceof the vacuum valve seat member 27 is hermetically sealed so as toprevent at least the flow of air which advances from a front end to arear end of the vacuum valve seat member 27. Further, a rear surface 27b and a front end surface 27 c of the flange 27 a of the vacuum valveseat member 27 are always communicated with the variable pressurechamber 9 and hence, the pressure of the variable pressure chamber 9 isalways applied to the rear surface 27 b and the front end surface 27 c.

Further, in a state that the vacuum valve portion 12 b of the valveelement 12 is seated on the vacuum valve seat 13 as shown in FIG. 3, anannular outer rear end surface 27 d of the vacuum valve seat member 27on an outer peripheral side than the sitting position of the vacuumvalve portion 12 b is always communicated with the constant pressurechamber 8 and hence, the pressure of the constant pressure chamber 8 isalways applied to the outer rear end surface 27 d. Further, in a statethat the vacuum valve portion 12 b of the valve element 12 is seated onthe vacuum valve seat 13, with respect to the vacuum valve seat member27, an annular inner rear end surface 27 e which is arranged on an innerperipheral side than the seated position of the vacuum valve portion 12b is communicated with the variable pressure chamber 9 and hence, thepressure of the variable chamber 9 is applied to the inner rear endsurface 27 e. Accordingly, during the operation of the negative pressurebooster 1, when a pressure difference arises between the variablechamber 9 and the constant pressure chamber 8, a force generated due tothe pressure difference is applied to the vacuum valve seat member 27 inthe rearward direction.

To a center of a front end portion of the valve body 4, as shown in FIG.2, a cylindrical holder 30 is integrally fixed with the valve body 4. Ona front end portion of the holder 30, an annular flange 30 a which comesinto contact with the reaction disc 23 is formed in an outwardlyprojecting manner. Further, on a rear end portion of the holder 30, aholder-side hook portion 30 b is formed in an outwardly projectingmanner.

The distance member 22 is slidably disposed in the inside of the holder30. During the inoperative state of the negative pressure booster 1,between a front end surface of the distance member 22 and a rear endsurface of the reaction disc 23 which faces the front end surface of thedistance member 22 in an opposed manner, a given gap C extending in theaxial direction is set.

A cylindrical member 29 is disposed in the inside of the axial-directionhole of the valve body 4. An annular flange 29 a which projectsoutwardly is formed on a rear end portion of the cylindrical member 29,while a cylindrical-member-side hook portion 29 b which is engageablewith a holder-side hook portion 30 b in the axial direction are formedon the front end portion of the cylindrical member 29 in an outwardlyprojecting manner. Further, between a flange 27 a of the vacuum valveseat member 27 and a flange 29 a of the cylindrical member 29, a spring31 having a spring constant K is disposed in a contracted manner and thecylindrical member 29 is always biased rearwardly due to a spring loadof the spring 31.

Then, during the usual operation, as shown in FIG. 2, since thecylindrical-member-side hook portion 29 b is axially engaged withholder-side hook portion 30 b, the further rearward movement of thecylindrical member 29 is prevented. Accordingly, the integral axialmovement of the holder 30 and the cylindrical member 29 with respect tothe valve body 4 is made impossible.

On the other hand, the vacuum valve seat member 27 is always biasedfrontwardly due to the spring load of the spring 31 and hence, as shownin FIG. 2, during the usual operation, the vacuum valve seat member 27is set at a first position where a portion of the front end of thevacuum valve seat member 27 is brought into contact with a bottomportion 4 b 1 of an inner hole 4 b of the valve body 4. Accordingly, thevacuum valve seat 13 one the rear end portion of the vacuum valve seatmember 27 is, during the usual operation, positioned at the firstposition shown in FIG. 2 with respect to the valve body 4. The vacuumvalve seat 13 which is positioned in the above-mentioned manner is setto assume the same state as a vacuum valve seat formed on a valve body 4of a conventional general negative pressure booster.

Further, the cylindrical-member-side hook portion 29 b is axiallyengaged with the holder-side hook portion 30 b and, at the same time,the front end of the vacuum valve seat member 27 is brought into contactwith the valve body 4. Further, in a state shown in FIG. 1, FIG. 2 andFIG. 4(a) in which a force attributed to the pressure difference is notapplied to the vacuum valve seat member 27, the spring load FS of thespring 31 is set to a set spring load FS0 which is preliminarilydetermined.

Further, when the input is applied to the input shaft 11 due to thestep-in operation of the brake pedal and the negative pressure booster 1is operated, the atmosphere is introduced into the variable pressurechamber 9 in the same manner as the conventional general negativepressure booster and hence, the pressure difference is generated betweenthe variable pressure chamber 9 and the constant pressure chamber 8.Accordingly, a force attributed to the pressure difference is applied tothe vacuum valve seat member 27 in the rearward direction. The force hasa magnitude which corresponds to the pressure difference between thevariable pressure chamber 9 and the constant pressure chamber 8, thatis, a magnitude of the input applied to the input shaft 11.

Then, assuming that the force attributed to the pressure difference isequal to or less than a sum of the above-mentioned set spring load FS0and the spring load fs of the valve spring 18 of the valve element 12 atthis point of time and the input which is applied to the input shaft 11is equal to or below a predetermined input F0 which is preliminarilydetermined, the vacuum valve seat member 27 is not moved with respect tothe valve body 4 and holds the first position shown in FIG. 2, FIG. 3and FIG. 4(b). Further, when the force attributed to the pressuredifference becomes larger than the sum of the set spring load FS0 andthe spring load fS and the input which is applied to the input shaft 11becomes larger than the predetermined input F0, the vacuum valve seatmember 27 is moved rearwardly relative to the valve body 4 and thecylindrical member 29 while pushing the vacuum valve portion 12 b of thevalve element 12. Accordingly, due to the rearward movement of thevacuum valve seat member 27, the vacuum valve seat 13 projectsrearwardly from the position during the usual operation.

In this case, as shown in FIG. 4(c) and FIG. 5(a), when a rear surfaceof the flange 27 a of the vacuum valve seat member 27 is brought intocontact with a front end 29 c of a cylindrical portion of thecylindrical member 29, the vacuum valve seat member 27 does not moverearwardly any more and is set at a second position. Accordingly, thevacuum valve seat 13 formed on the rear end portion of the vacuum valveseat member 27 is, when the input is larger than the predetermined inputF0, positioned at the second position shown in FIG. 4(c) and FIG. 5(a)with respect to the valve body 4. A stroke quantity of the vacuum valveseat member 27 during this movement is within the range of a gap L (Lbeing shown in FIG. 2) between a rear surface of the flange 27 a and afront end 29 c of the cylindrical portion of the cylindrical member 29when the negative pressure booster 1 shown in FIG. 1, FIG. 2 and FIG.4(a) is not operated.

Here, when the vacuum valve seat member 27 strokes rearwardly relativeto the valve body 4, an atmospheric valve portion 12 a of theatmospheric valve 16 also strokes rearwardly relative to the valve body4 by an amount equal to a relative stroke quantity L″ of the vacuumvalve seat member 27. Accordingly, a valve opening quantity between theatmospheric valve portion 12 a and the atmospheric valve seat 14 is,compared to the case in which it is assumed that the vacuum valve seatmember 27 has no relative stroke, increased by an amount correspondingto the relative stroke quantity L″ of the vacuum valve seat member 27provided that the input stroke quantity of the input shaft 11 is equal.That is, in an intermediate load state in which both of the vacuum valve15 and the atmospheric valve 16 are closed and balanced, when the inputstroke quantity of the input shaft 11 is equal, the respective strokesof the valve body 4 and the piston member 6 of the power piston 5 areincreased by an amount corresponding to the relative stroke quantity L″of the vacuum valve seat member 27 compared with the case in which it isassumed that the vacuum valve seat member 27 has no relative movement.In other words, assuming that the respective stroke quantities of thevalve body 4 and the piston member 6 of the power piston 5 are equalbetween the case in which the vacuum valve seat member 27 has therelative stroke and the case in which it is assumed that the vacuumvalve seat member 27 has no relative stroke, the case in which thevacuum valve seat member 27 has the relative stroke allows the stroke ofthe input shaft 11 to be shortened by an amount corresponding to therelative stroke quantity L″ of the vacuum valve seat member 27.

On the other hand, an output stroke of the output shaft 24 during theabove-mentioned relative stroking of the vacuum valve seat member 27 isalso, assuming that the input stroke quantity of the input shaft 11 isequal in the above-mentioned manner, increased along with the increaseof respective strokes of the valve body 4 and the piston member 6 of thepower piston 5. However, in the intermediate load condition, as shown inFIG. 4(c), the reaction disc 23 bulges toward the distance member 22 anda thickness of the reaction disc 23 in the axial direction is reducedand hence, the output stroke of the output shaft 24 becomes smaller thanthe increased relative stroke quantity L″ of the above-mentionedrespective strokes of the valve body 4 and the piston member 6 of thepiston 5. Accordingly, an increase quantity L′ of the output stroke ofthe output shaft 24 is given by a following formula as shown in FIG. 6.L′=L″×[1−(1/SR1)]  . . . (1)

Here, SR1 indicates a servo ratio SR1 in the low deceleration (low G)region.

To explain a process for obtaining the formula (1), when the brakingmanipulation is performed in the intermediate deceleration (intermediateG) region, the reaction disc 23 bulges toward the distance member 22 anda thickness thereof in the axial direction is decreased as mentionedabove. Here, the axially decreased quantity of the thickness is assumedas L1. Further, assuming a cross-sectional area of the distance member22 as A1 and a cross-sectional area of the reaction disc 23 as A2, therespective strokes of the valve body 4 and the piston member 6 of thepower piston 5 are increased by an amount corresponding to the strokequantity L″ as described above and hence, a following relationship isestablished.L″×A1=L1×A2  . . . (2)

Here, since the servo ratio SR1 is (A2/A1) and hence, by modifying theformula (2) with respect to L1, a following relationship is established.L1=L″×(1/SR1)   . . . (3)

That is, along with the bulging of the reaction disc 23, the axialthickness of the reaction disc 23 is decreased by an amount of L″/SR1.Accordingly, the increased stroke quantity L′ of the output stroke ofthe output shaft 24 is expressed by a following formula (4).L′=L″−(L″/SR1)=L″×[1−(1/SR1)]  . . . (4)

Then, in the intermediate deceleration (intermediate G) region whichconstitutes an intermediate load state shown in FIG. 6, assuming thatthe stroke of the output shaft 24 has an equal stroke quantity a betweenthe case in which the vacuum valve seat member 27 has the relativestroke (indicated by a solid line in FIG. 6) and the case in which it isassumed that the vacuum valve seat member 27 has no relative stroke(indicated by a dotted line), the case in which the vacuum valve seatmember 27 has the relative stroke allows the stroke of the input shaft11 to be shortened by a stroke quantity β.

Here, the negative pressure booster 1 of this embodiment is set suchthat immediately before the rear surface of the flange 27 a of thevacuum valve seat member 27 comes into contact with the front end 29 cof the cylindrical portion of the cylindrical member 29, the negativepressure booster 1 assumes a full load state, that is, the pressure ofthe variable pressure chamber 9 which is applied to the vacuum valveseat member 27 assumes the atmospheric pressure. Accordingly, themaximum stroke quantity L″max of the relative stroke quantity L″ of thevacuum valve seat member 27 relative to the valve body 4 during theabove-mentioned relative movement of the vacuum valve seat member 27becomes slightly smaller than the gap L (L″max<L). That is, the maximumshortened stroke quantity of the input shaft 11 is given by L″max.

Further, as shown in FIG. 6, the maximum stroke quantity L′ max whichincreases the output stroke of the output shaft 24 in this case isexpressed by a following formula (5).L′max=L″max×[1−(1/SR1)]  . . . (5)

Then, since the vacuum valve seat member 27 projects rearwardly whilepushing the vacuum valve portion 12 b of the valve element 12, the valveelement 12 moves rearwardly and the atmospheric valve portion 12 a ofthe valve element 12 also moves in the rearward direction. Accordingly,the atmospheric valve portion 12 a is further largely separated from theatmospheric valve seat 14 than a state shown in FIG. 3 and FIG. 4(b) inwhich the atmospheric valve 16 is closed during the usual brakingoperation. That is, the vacuum valve seat member 27 is configured toincrease a valve opening quantity of the atmospheric valve 16. In thismanner, the stroke shortening mechanism, that is, the atmospheric valvevalve-opening-quantity increasing means of the present invention isconstituted of the vacuum valve seat member 27 and the spring 31, andthe operation of the atmospheric valve valve-opening-quantity increasingmeans is controlled in response to the pressure difference between thevariable pressure chamber 9 and the constant pressure chamber 8.

The movement of the vacuum valve seat member 27 is specificallyexplained. Here considered is a force attributed to the pressuredifference which is applied to the vacuum valve seat member 27 in theintermediate load state in which, as shown in FIG. 5(a), the vacuumvalve seat member 27 moves and both of the vacuum valve 15 and theatmospheric valve 16 are closed so that the control valve 17 assumes abalanced state. Here, the balanced state of the control valve 17 shownin FIG. 5(a) is, considered to be the state in which since the vacuumvalve seat member 27 and the valve element 12 are brought into contactwith each other and are integrally formed forces applied to the vacuumvalve seat member 27 and the valve element 12 which are integrallyformed with each other as shown in FIG. 5(b) are equivalent.

Here, in FIG. 5(b), assume the force attributed to the pressuredifference between the pressures applied to the vacuum valve seat member27 and the valve element 12 as FP, the pressure of the constant pressurechamber 8 as PV0, the pressure of the variable pressure chamber 9 as PV,the atmospheric pressure as Pa, an effective pressure receiving area ofthe annular front end surface 27 c of the vacuum valve seat member 27which receives the variable-pressure-chamber pressure PV as AL, aneffective pressure receiving area of the annular rear end surface 27 einside a sitting point of the vacuum valve seat member 27 at the time ofsitting of the vacuum valve portion on the vacuum valve seat member 27and the rear surface 27 b of the flange 27 a which receives thevariable-pressure-chamber pressure PV as AV, and an effective pressurereceiving area of the valve element 12 which receives the atmosphericpressure Pa as AP. Further, assume that the relationship AP≈AV is setand, at the same time, a diameter of the sitting position of theatmospheric valve portion 12 a on the atmospheric valve seat 14 is setto substantially agree with an effective diameter of the effectivepressure receiving area AP of the valve element 12. In this case, theforce FP attributed to the pressure difference between the pressuresapplied to the vacuum valve seat member 27 and the valve element 12 isexpressed by a following formula (6).FP=(PV−PV0)·(AL−AV)   . . . (6)

Here, this force FP pushes the vacuum valve seat member 27 and the valveelement 12 rearwardly. on the other hand, the spring load FS of thespring 31 and the spring load fS of the valve spring 18 push the vacuumvalve seat member 27 and the valve element 12 frontwardly. Accordingly,when the above-mentioned force FP becomes larger than the sum of thesespring loads (FS+fS), the vacuum valve seat member 27 moves rearwardly.Here, an absolute value of the spring load fS of the valve spring 18 issmall and, at the same time, is set to an extremely small value comparedwith the spring load FS of the spring 31 (FS>>fS) and hence, when theforce FP is substantially larger than the spring load FS (FP>FS), thevacuum valve seat member 27 moves rearwardly, while when the force FP isequal to or below the spring load FS (FP≦FS), the vacuum valve seatmember 27 does not move rearwardly.

Then, when the pressure of the variable pressure chamber 9 is elevatedand the force FP becomes larger than the set spring load FS0, the vacuumvalve seat member 27 starts the rearward movement. The pressure PV ofthe variable pressure chamber 9 when the vacuum valve seat member 27moves is expressed by a following formula (7).PV>[FS/(AL−AV)]+PV0   . . . (7)

In this case, it is needless to say that it is necessary to set therelationships AL>AV and AL>AP to move the vacuum valve seat member 27.

A region of the pressure PV of the variable pressure chamber 9 whichdoes not satisfy the formula (7) is an output region in which the outputof the negative pressure booster 1 is equal to or less than the givenoutput F1 in the input-output characteristics shown in FIG. 7. In thisregion, the input is relatively small, and the deceleration attributedto braking is equal to the deceleration during the usual brakingoperation in the conventional vehicle of relatively low weight(including the carrying weight), wherein the region is set as a lowdeceleration (low G) region with respect to the relatively high weight(including the carrying weight) vehicle. This low deceleration (low G)region is the usual braking operation region. In this low G region, theservo ratio is set to a relatively small servo ratio SR1 substantiallyequal to the servo ratio at the time of performing the conventionalbraking operation.

Further, the region of pressure PV of the variable pressure chamber 9which satisfies the formula (7) is an output region in which the outputof the negative pressure booster 1 is larger than the given output F1.In this region, the input is relatively large and the decelerationattributed to the braking is set as the intermediate deceleration(intermediate G) region with respect to the vehicle of relatively highweight (including the carrying weight). In this intermediatedeceleration (intermediate G) region, the vacuum valve seat member 27projects rearwardly and pushes the valve element 12 rearwardly. As theresult, the valve opening quantity of the atmospheric valve 16 isincreased compared with the usual braking operation time with the sameinput and hence, the servo ratio becomes a servo ratio SR2 which islarger than the servo ratio SR1 of the conventional usual brakingoperation time in which the servo ratio is set in the low decelerationspeed (low G) region (SR2>SR1).

The servo ratio SR2 is explained in detail. In the negative pressurebooster 1 of this embodiment, the serve ratio SR2 is obtained in afollowing manner. That is, the valve opening quantity of the atmosphericvalve 16 is slightly increased as described above so as to elevate thepressure of the variable pressure chamber 9 thus slightly jumping up theoutput. Then, by repeating this jumping up of the output in a state ofservo ratio SR1 thus elevating the output in a step-like manner with asmall step quantity microscopically, it is possible to obtain the servoratio SR2 which is larger than servo ratio SR1 in appearancemacroscopically.

Here, in the negative pressure booster 1 of this embodiment, both of aspring constant K and the set spring load FS0 of the spring 31 whichbiase the vacuum valve seat member 27 can be arbitrarily set. Further,in the input-output characteristics of the negative pressure booster 1of this embodiment shown in FIG. 7, the set input F0 which is an inputof a change point (ratio point) γ at which the servo ratio is changedfrom the small servo ratio SR1 to the large servo ratio SR2 can beshifted upwardly and downwardly by changing the set spring load FS0 ofthe spring 31. Further, the servo ratio SR can be increased or decreasedby changing the spring constant K of the spring 31. Further, theabove-mentioned gap L also can be arbitrarily determined.

Accordingly, in the negative pressure booster 1 of this embodiment, bysetting the spring constant K and the set spring load FS0 of the spring31 and the stroke quantity L of the vacuum valve seat member 27corresponding to a vehicle on which the negative pressure booster 1 ismounted, it is possible to easily and properly apply the negativepressure booster 1 of one type to brake boosters of various kinds ofvehicles corresponding to the types of vehicles.

Next, the manner of operation of the negative pressure booster 1 of thisexample is explained.

Negative Pressure Booster in Operative State

The negative pressure is always introduced into the constant pressurechamber 8 of the negative pressure booster 1 through the negativepressure introducing passage 26. Further, in the inoperative state ofthe negative pressure booster 1 shown in FIG. 1 and FIG. 2, the keymember 21 is brought into contact with the rear shell 3 to assume aretraction limit. Accordingly, the valve body 4 and a valve plunger 6assume the retraction limit due to the key member 21 and, further, thepower piston 5, the input shaft 11 and the output shaft 24 also assume aretraction limit. In this inoperative state, the atmospheric valveportion 12 a of the valve element 12 is seated on the atmospheric valveseat 14 so as to close the atmospheric valve 16, while the vacuum valveportion 12 b of the valve element 12 is separated from the first vacuumvalve seat 13 and the second vacuum valve seat 27 g so as to open thevacuum valve 15. Accordingly, the variable pressure chamber 9 isinterrupted from the atmosphere and is communicated with the constantpressure chamber 8 and hence, the negative pressure is introduced intothe variable pressure chamber 9 whereby no substantial pressuredifference is generated between the variable pressure chamber 9 and theconstant pressure chamber 8.

Accordingly, the force attributed to the pressure difference is notrearwardly applied to the vacuum valve seat member 27 and hence, thevacuum valve seat member 27 is positioned at the position shown in FIG.2 where a portion of the front end face 27 c is brought into contactwith the bottom portion 4 b 1 of the inner hole 4 b of the valve body 4.

The Negative Pressure Booster During Usual Braking Operation in LowDeceleration Region

When the brake pedal is stepped in at the step-in speed during the usualbraking operation to perform the usual braking, the input shaft 11advances and hence, the valve plunger 10 advances. Along with theadvancing of the valve plunger 10, the vacuum valve portion 12 b of thevalve element 12 is seated on the vacuum valve seat 13 so as to closethe vacuum valve 15 and, at the same time, the atmospheric valve seat 14is separated from the atmospheric valve portion 12 a of the valveelement 12 so as to open the atmospheric valve 16. That is, the variablepressure chamber 9 is interrupted form the constant pressure chamber 8and, at the same time, is communicated with the atmosphere. Accordingly,the atmosphere is introduced into the variable pressure chamber 9through the atmosphere introducing passage 19 and the opened atmosphericvalve 16. As a result, the pressure difference is generated between thevariable pressure chamber 9 and the constant pressure chamber 8 andhence, the power piston 5 advances. Further, the output shaft 24advances by way of the valve body 4 and hence, the piston of the mastercylinder not shown in the drawing advances.

Further, although the distance member 22 also advances along with theadvancing of the valve plunger 10, the distance member 22 is not yetbrought into contact with the reaction disc 23 due to the gap C.Accordingly, the reaction from the output shaft 24 is not transmitted tothe distance member 22 from the reaction disc 23 and hence, the reactionis not also transmitted to the brake pedal by way of the valve plunger10 and the input shaft 11. When the input shaft 11 further advances, thepower piston 5 also further advances, and the piston of the mastercylinder further advances by way of the valve body 4 and the outputshaft 24.

When a lost stroke of the brake system following the master cylinder iseliminated, the negative pressure booster 1 substantially generates anoutput and hence, the master cylinder generates a master cylinderpressure (hydraulic pressure) due to such an output, and the wheelcylinder is operated with the master cylinder pressure and the brakingforce is generated.

Here, due to the reaction applied to the output shft 24 from the mastercylinder, the reaction disc 23 bulges rearwardly as shown in FIG. 3 andFIG. 4(b) and hence, the gap C is eliminated and the reaction disc 23 isbrought into contact with the distance member 22. Accordingly, thereaction from the output shaft 24 is transmitted to the distance member22 from the reaction disc 23 and, further, the reaction is transmittedto the brake pedal by way of the valve plunger 10 and the input shaft 11and is detected or perceived by a driver. That is, as shown in FIG. 7,the negative pressure booster 1 exhibits the jumping characteristicsduring the usual braking operation. This jumping characteristics aresubstantially equal to the jumping characteristics of the conventionalgeneral negative pressure booster.

When the usual braking is performed within the low deceleration (low G)region, the input of the negative pressure booster 1 attributed to thepedal step-in force is relatively small. This low deceleration (low G)region is the output region where the output is equal to or below thegiven output and, as mentioned above, the pressure PV of the variablepressure chamber 9 does not satisfy the formula (7). Accordingly, thevacuum valve seat member 27 does not move and the servo ratio is set tothe relatively small servo ratio SR1 which is substantially equal to theservo ratio during the conventional usual braking operation.Accordingly, when the output of the negative pressure booster 1 has themagnitude which is obtained by boosting the input of the input shaft 11due to the pedal step-in force with the servo ratio SR1, the atmosphericvalve portion 12 a is seated on the atmospheric valve seat 14 thus alsoclosing the atmospheric valve 16 whereby the balanced state of theintermediate load is established (the vacuum valve 15 is already closeddue to the seating of the vacuum valve portion 12 b on the vacuum valveseat 13). In this manner, in the low deceleration (low G) region shownin FIG. 7, the usual braking is performed with the brake force which isobtained by boosting the pedal step-in force during the usual brakingoperation with the servo ratio SR1.

When the brake pedal is reduced to release the usual braking from astate in which both of the atmospheric valve 16 and the vacuum valve 15of the negative pressure booster 1 are closed during the usual brakingoperating shown in FIG. 3 and FIG. 4(b), both of the input shaft 11 andthe valve plunger 10 retract. However, since air (atmosphere) isintroduced into the variable pressure chamber 9, the valve body 4 andthe vacuum valve seat member 27 do not retract immediately. Accordingly,the atmospheric valve seat 14 of the valve plunger 10 pushes theatmospheric valve portion 12 a of the valve element 12 rearwardly andhence, the vacuum valve portion 12 b is separated from the vacuum valveseat 13 g so as to open the vacuum valve 15. Then, the variable pressurechamber 9 is communicated with the constant pressure chamber 8 by way ofthe opened vacuum valve 15 and the vacuum passage 20 and hence, airintroduced into the variable pressure chamber 9 is discharged to avacuum source by way of the opened vacuum valve 15, the vacuum passage20, the constant pressure chamber 8 and the negative pressureintroducing passage 26.

Accordingly, the pressure of the variable pressure chamber 9 is loweredand the pressure difference between the variable pressure chamber 9 andthe constant pressure chamber 8 becomes small and hence, due to thespring force of the return spring 25, the power piston 5, the valve body4 and the output shaft 24 retract. Along with the retracting of thevalve body 4, due to a spring force of a return spring of the piston ofthe master cylinder, the piston of the master cylinder and the outputshaft 24 retract and hence, the release of the usual braking is started.

When the key member 21 is brought into contact with the rear shell 3 asshown in FIG. 1, the key member 21 is stopped and does not retractfurther. However, the valve body 4, the vacuum valve seat member 27, thevalve plunger 10 and the input shaft 11 retract further. Then, the valveplunger 10 is brought into contact with the key member 21 as shown inFIG. 2 and does not retract further. Further, the front end 4 a 1 of thekey groove 4 a of the valve body 4 is brought into contact with the keymember 21 as shown in FIG. 2 and the valve body 4 does not retractfurther. In this manner, the negative pressure booster 1 assumes theinitial inactive state shown in FIG. 1, FIG. 2 and FIG. 4(a).Accordingly, the master cylinder assumes the inoperative state andhence, the master cylinder pressure is eliminated and, at the same time,the wheel cylinder becomes inoperative and the braking force iseliminated whereby the usual braking is released.

The Negative Pressure Booster During Usual Braking Operation inIntermediate Deceleration Region

In performing the usual braking operation in the intermediatedeceleration region which exhibits the larger deceleration than the lowdeceleration (low G) during the usual braking operation, the input ofthe negative pressure booster 1 by the pedal step-in operation is setlarger than the input during the usual braking operation in the lowdeceleration (low G) region. Although the pressure Pv of the variablepressure chamber 9 is increased along with the increase of the input tothe negative pressure booster 1, when the input becomes a value equal toor above the predetermined input F0 in which the pressure PV of thevariable pressure chamber 9 satisfies the formula (7) in FIG. 7, theinput-output characteristics of the negative pressure booster 1 assumethe intermediate deceleration (intermediate G) region which is an outputregion in which the output becomes larger than the given output.

In this intermediate deceleration (intermediate G) region, the pressurePv of the variable pressure chamber 9 satisfies the formula (7) andhence, the vacuum valve seat member 27 moves rearwardly while pushingthe valve element 12. Accordingly, the atmospheric valve portion 12 a islargely separated from the atmospheric valve seat 14 than the usualoperation time and hence, the atmospheric valve 16 is largely opened.Accordingly, in the intermediate G region shown in FIG. 7, the serveratio assumes the servo ratio SR2 which is larger than the servo ratioduring the conventional usual braking operation described previously.That is, when the output of negative pressure booster 1 becomes themagnitude obtained by boosting the input of the input shaft 11 by theserve ratio SR2, in the same manner as mentioned previously, theatmospheric valve portion 12 a is seated on the atmospheric valve seat14 and the atmospheric valve 16 is closed thus establishing the balancedstate of the intermediate load (the vacuum valve 15 being already closedas the vacuum valve portion 12 b is seated on the vacuum valve seat 13).In this manner, in the intermediate deceleration (intermediate G)region, the braking is performed with the braking force which isobtained by boosting the pedal step-in force by servo ratio SR2 and islarger than the braking force during the usual braking operation in thelow deceleration (low G). In this case, the negative pressure booster 1can obtain, in the intermediate deceleration (intermediate G) region,the output larger than the output during the usual braking operationusing the pedal step-in force, that is, the large input of the negativepressure booster 1 and is equal to the input during the usual brakingoperation by servo ratio SR1.

Further, during the operation in the intermediate deceleration(intermediate G) region, the vacuum valve seat member 27 movesrearwardly by the stroke quantity L″ compared to the operation in thelow deceleration (low G) region and hence, the output stroke isincreased corresponding to the stroke quantity L. In other words, asshown in FIG. 6, to obtain the same stroke α, the input stroke quantityin the intermediate deceleration (intermediate G) region indicated by asolid line in FIG. 7 is made smaller than the input stroke quantity inthe low deceleration (low G) region indicated by a dotted line when theoutput stroke is changed at a change rate (inclination) of the inputstroke relative to the output stroke during the usual braking operationwith the servo ratio SR1 by a stroke quantity β whereby the stroke ofthe input shaft 11, that is, the stroke of the brake pedal can beshortened.

By releasing the brake pedal to release the usual braking from a statein which the atmospheric valve 16 and the vacuum valve 15 of thenegative pressure booster 1 are closed during the operation of thevacuum valve seat member 27 shown in FIG. 4(c) and FIG. 5(a), the vacuumvalve 15 is opened in the same manner as mentioned above, and air whichis introduced in the variable pressure chamber 9 is discharged to thevacuum source by way of the opened vacuum valve 15, the vacuum passage20, the constant pressure chamber 8 and the negative pressureintroducing passage 26.

Accordingly, the pressure of the variable pressure chamber 9 is loweredin the same member as described above and due to the spring force of thereturn spring 25, the power piston 5, the valve body 4 and the outputshaft 24 retract. Along with the retracting of the valve body 4, due toa spring force of a return spring of the piston of the master cylinder,the piston of the master cylinder and the output shaft 24 retract andhence, the release of the usual braking is started.

When the pressure Pv of the variable pressure chamber 9 does not satisfythe formula (7), due to the spring load FS of the spring 31, the vacuumvalve seat member 27 moves frontwardly relative to the valve body 4 andhence, the vacuum valve seat member 27 assumes the inoperative positionshown in FIG. 2. Accordingly, the vacuum valve portion 12 b is largelyseparated from the vacuum valve seat 13 g and the vacuum valve 15 islargely opened. Accordingly, air in the variable pressure chamber 9 islargely discharged and hence, the negative pressure booster 1 assumesthe usual brake operating state in the low deceleration (low G) region.Thereafter, the operation is performed in the same manner as theabove-mentioned usual braking operation in the low deceleration (low G).Finally, all of the moved members of the negative pressure booster 1assume the inoperative position shown in FIG. 2 and hence, the brakingattributed to the input larger than the braking during the usual brakingoperation in the low deceleration (low G) region is released.

In this manner, according to the negative pressure booster 1 which isapplied to the brake system, in obtaining the large stroke of the outputshaft 24 in the intermediate deceleration (intermediate G) region, it ispossible to shorten the stroke quantity of the input shaft 1 than thestroke quantity necessary for obtaining the large stroke when the strokequantity of the input shaft 11 is changed at a change rate of themanipulation stroke quantity of the above-mentioned input shaft withrespect to the output in the low deceleration (low G) region.Accordingly, to obtain the deceleration larger than the decelerationduring the usual braking operation in the low deceleration (low G)region, it is possible to obtain the desired large deceleration with apedal step-in quantity which is smaller than the step-in quantity of thebrake pedal necessary for obtaining the large deceleration with theservo ratio SR1 during the usual braking operation in the lowdeceleration (low G) region. Accordingly, it is possible to moreeffectively obtain the favorable brake feeling with respect to thevehicle which requires the large braking force during the usual brakingoperation in the intermediate deceleration (intermediate G) region thanduring the usual braking operation in the low deceleration (low G)region of a vehicle which has the large vehicle weight.

Here, in the above-mentioned embodiment, the negative pressure booster 1assumes the full load state immediately before the rear surface of theflange 27 a of the vacuum valve seat member 27 is brought into contactwith the front end 29 c of the cylindrical portion of the cylindricalmember 29. However, it is possible to allow the negative pressurebooster 1 to assume the full load state at a point of time that the rearsurface of the flange 27 a of the vacuum valve seat member 27 is broughtinto contact with the front end 29 c of the cylindrical portion of thecylindrical member 29. Alternatively, it is possible to allow thenegative pressure booster 1 to assume the full load state after the rearsurface of the flange 27 a of the vacuum valve seat member 27 is broughtinto contact with the front end 29 c of the cylindrical portion of thecylindrical member 29. In these cases, the maximum shortened strokequantity of the input shaft 11 becomes L.

Further, in the above-mentioned embodiment, the operation control of thevacuum valve seat member 27 is performed based on the pressuredifference between the pressure of the variable pressure chamber 9 andthe pressure of the constant pressure chamber. However, the presentinvention is not limited to such an operation control and the operationcontrol of the vacuum valve seat member 27 may be performed based ononly the pressure of the variable pressure chamber 9 or the pressuredifference between the variable pressure chamber 9 and another fixedpressure. Further, in place of the pressure of the variable pressurechamber 9, the operation of the vacuum valve seat member 27 may becontrolled in response to a pressure corresponding to an input appliedto the input shaft.

Further, in the above-mentioned embodiment, the present invention isapplied to the single-type negative pressure booster which includes onepower piston 5. However, the present invention is applicable to atandem-type negative pressure booster which includes a plurality ofpower pistons 5.

Still further, although the negative pressure booster of the presentinvention is applied to the brake system in the previously-mentionedembodiment, the present invention is applicable to other system ordevice which uses the negative pressure booster.

INDUSTRIAL APPLICABILITY

The negative pressure booster of the present invention is preferablyapplicable to a booster in a boosting system such as a brake booster ina brake boosting system of an automobile.

1. A negative pressure booster comprising at least a valve body which isreciprocally disposed in the inside of a shell, a power piston which ismounted in the valve body and partitions the interior of the shell intoa constant pressure chamber into which negative pressure is introducedand a variable pressure chamber into which atmosphere is introduced atthe time of operating the negative pressure booster, a valve plungerwhich is connected to an input shaft and is slidably disposed in theinside of the valve body, a vacuum valve which controls thecommunication or the interruption between the constant pressure chamberand the variable pressure chamber, and an atmospheric valve whichcontrols the interruption or the communication between the variablepressure chamber and at least the atmosphere due to the operation of thevalve plunger, wherein the negative pressure booster further includes astroke shortening mechanism which shortens a manipulation strokequantity of the input shaft in an output region with an output largerthan a given output than the manipulation stroke quantity of the inputshaft in the output region with the output larger than the given outputwhen the manipulation stroke quantity of the input shaft is changed at achange rate of the manipulation stroke quantity of the input shaft withrespect to the output in an output region with an output equal to orbelow the given output.
 2. A negative pressure booster according toclaim 1, wherein the stroke shortening mechanism is atmospheric valveopening quantity increasing means which is operated in the output regionwith the output larger than the given output and increases a valveopening quantity of the atmospheric valve larger than the valve openingquantity during the usual operation, and the operation of theatmospheric valve opening quantity increasing means is controlled inresponse to pressure corresponding to the input.
 3. A negative pressurebooster according to claim 2, wherein the pressure which controls theoperation of the atmospheric valve opening quantity increasing means ispressure of the variable pressure chamber.
 4. A negative pressurebooster according to claim 3, wherein the vacuum valve includes a valveelement and a vacuum valve seat on which the valve element is detachablyseated and, the atmospheric valve includes the valve element and anatmospheric valve seat on which the valve element is detachably seated,and the atmospheric valve opening quantity increasing means includes avalve seat member which has the vacuum valve seat mounted on one endside thereof, wherein the valve seat member is mounted in the valve bodymovably between a first position which is positioned in the outputregion with the output equal to or below the given output and a secondposition which is positioned in the output region with the output largerthan the given output, and the movement of the valve seat member iscontrolled in response to the pressure of the variable pressure chamber.5. A negative pressure booster according to claim 4, wherein themovement of the valve seat member is controlled in response to thepressure difference between the variable pressure chamber and theconstant pressure chamber.